1. Field of the Invention
This invention relates to a method for production of a magnetic bubble memory device, especially, of the type provided with magnetic bubble propagation circuit formed by ion implantation. Such a device will be termed an ion-implanted device hereinafter.
2. Description of the Prior Art
As well known in the art, it has hitherto been general practice to use a magnetic bubble memory device with permalloy-film magnetic bubble propagation circuit, that is to say, a so-called permalloy device.
Particularly, in the permalloy device, a permalloy (soft magnetic substance) film 1 having a planar pattern, for example, as shown in FIG. 1 is provided on a magnetic bubble holding film (not shown) of, for example, magnetic garnet (YSmLuCa).sub.3 (FeGe).sub.5 O.sub.12 to form magnetic bubble propagation circuit, and a rotating magnetic field is applied parallel to the garnet film to propagate a magnetic bubble 2.
The permalloy pattern (permalloy film) 1 and a conductor pattern 5 of, for example, an Al-Cu or Au film formed between a magnetic garnet film 3 and the permalloy pattern 1 through insulating films 4 and 6, as shown in partial sectional form in FIG. 2, constitute a bubble generator, a transfer gate, a swap gate or a replicator adapted to generate, transfer, swap or replicate magnetic bubbles. When a control pulse current is passed through the conductor pattern 5, various functions such as generation of the magnetic bubble and transfer thereof are carried out.
Typically, the magnetic garnet film 3 for holding the magnetic bubbles is formed through liquid phase epitaxial growth process on a (111) oriented surface of a non-magnetic single crystalline substrate of, for example, Gd.sub.3 Ga.sub.5 O.sub.12. The non-magnetic substrate, however, is not directly related to the present invention and is not depicted in FIG. 2 to avoid prolixity of illustration.
With the progress of high-density and highly integrated formation of the magnetic bubble device, highly fine patterning of the permalloy propagation circuit has been employed wherein the width and gap of the permalloy pattern are considerably reduced. For example, in order to form a device of a bit period of 8 .mu.m using the magnetic bubble having a diameter of about 2 .mu.m, the permalloy pattern is required to have a width and a gap of about 1 .mu.m.
Moreover, materialization of a future permalloy device which is further advanced in density will require the accurate formation of a fine pattern of less than 1 .mu.m width and gap over the entire chip. Existing technique is, however, difficult to meet such a requirement.
To cope with this problem, a new type of magnetic bubble memory device has recently been proposed as disclosed in U.S. Pat. No. 3,828,329 and it has been highlighted.
This type of magnetic bubble memory device advantageously substitutes a propagation circuit formed by ion implantation for the conventional propagation circuit made of a film of soft magnetic substance such as permalloy, and it is called an ion-implanted device.
More particularly, as schematically shown in FIG. 3, a mask in the form of a contiguous disc (not shown) is applied to cover a desired portion of the magnetic garnet film 3 and various ions such as for example H.sup.+, H.sub.2.sup.+, D.sub.2.sup.+, He.sup.+ and Ne.sup.+ are implanted into exposed portions of the magnetic garnet film to form ion-implanted regions 7 outside the mask so that magnetization in the regions 7 directs parallel to the film plane.
When a rotating magnetic field is applied parallel to the magnetic garnet film having the ion-implanted regions, the magnetic bubble is propagated along the edge of a contiguous-disc region (propagation circuit) 16 as will be done along the permalloy pattern in the conventional device.
With the ion-implanted device, the propagation circuit 16 can advantageously have a pattern size which is about twice as large as that of the permalloy pattern for obtaining the same bit density. This ensures that the ion-implanted device can be easy to produce and can be highly suitable for high-density formation.
The ion-implanted device makes use of properties of a magnetized layer parallel to the magnetic garnet film which is set up on account of magnetostrictive effect due to ion implantation. In particular, as shown in FIG. 4, greater ion implantation effect can be obtained by ion implantation with hydrogen ion than by ion implantation with other ions, and an anisotropic magnetic field .DELTA.Hk parallel to the magnetic garnet film can be increased by increasing the ion dose.
For the sake of obtaining a desired amount of magnetostriction, the ion implantation with hydrogen ion is disadvantageous because the small mass of hydrogen ion requires the ion dose to be increased considerably and because hydrogen ion liable to volatilize at high temperatures makes characteristics unstable when heat treatment is effected after the ion implantation.
For these reasons, a method of multiple ion implantation has been proposed wherein hydrogen ion is combined with thermally stable ions such as Ne.sup.+ and He.sup.+.
A magnetic garnet film prepared by this method, however, suffers from a low Curie temperature and has difficulties for practical use. Thus, the advent of a solution to the above problems has been desired strongly.